This invention relates to an electrophoretic medium with improved stability. More specifically, it relates to an electrophoretic medium which contains a stable free radical or polymeric radical scavenger to improve the stability of the medium. The electrophoretic medium also desirably contains an ultra-violet absorber and/or quencher.
Electrophoretic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. (The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element.) Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
An encapsulated, electrophoretic display typically does not suffer to the same degree from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
However, even encapsulated prior art electrophoretic displays have working lifetimes less than is entirely desirable for commercial purposes. Depending upon operational parameters, such as the voltage applied to the electrophoretic medium and the frequency at which any given pixel of the display is switched, and environmental parameters, such as the temperature, humidity and light intensity under which the electrophoretic display is operated, various components of the electrophoretic medium may be subject to electrochemical and/or photochemical reactions, which may cause problems such as aggregation of the electrophoretic particles or discoloration of a dye present in the liquid suspension medium.
Previous attempts have been made to solve these problems in unencapsulated displays. For example, U.S. Pat. No. 4,620,916 (Zwemer et al.) notes that “there have been problems in realizing commercially successful display devices based upon electrophoretic principles. Among the problems have been the difficulty of achieving sufficient stability in the electrophoretic cell so as to sustain an acceptable number of switching operations. Chemical reactions in the suspension of the cell adversely affect operating lifetime and the known electrophoretic display devices have generally had a limited useful life.” In an attempt to solve such problems Zwemer teaches that there should be added to the working liquid of the electrophoretic display a “degradation retardant [which] consists essentially of a redox agent, a redox agent precursor, an inhibitor agent or mixtures thereof. The redox agent is capable of being reversibly oxidized and reduced during operation of the electrophoretic display device. The redox agent precursor is capable of reacting with free radical species of the working liquid to form the redox agent as a reaction product. The inhibitor agent is capable of reacting with free radical species to terminate chemical chain reactions during operation of the electrophoretic display device.”
Although the degradation retardants disclosed in Zwemer do relieve some of the problems discussed above, these retardants are not entirely satisfactory. In particular, the specific degradation retardants disclosed in Zwemer do not absorb ultra-violet light and are not effective in preventing light-induced degradation of an electrophoretic medium. Also, the Zwemer retardants cause problems in encapsulated electrophoretic media since under high humidity conditions they may diffuse through the capsule wall into the binder which is normally used in an encapsulated electrophoretic medium to bind the capsules to one another and cause degradation (browning) of this binder or discoloration of the electrodes of the electrophoretic display.
It has now been found that the problems discussed above can be reduced, and the working lifetime of electrophoretic media and of displays incorporating such media increased, by adding to the electrophoretic medium either a stable free radical or a polymeric free radical scavenger.